1. Field of the Invention
The present invention relates to a laser interferometer, more particularly to a laser interferometer with a laser source that emits a laser beam while stabilizing a center wavelength of the laser beam by modulating the laser beam using a modulation signal of a predetermined frequency.
2. Description of Related Art
A traditionally-known Michelson-type interferometer includes: a laser source for irradiating a laser beam; a reference surface provided at a predetermined position for reflecting the laser beam; and a measurement surface provided on a target object for reflecting the laser beam. The Michelson-type interferometer measures a displacement of the target object by calculating a displacement of the measurement surface based on interference light reflected by the reference surface and the measurement surface (for example, Document 1: JP-A-02-22503).
Such an interferometer includes a sampling unit for acquiring a sampling value by converting the intensity of the interference light into an electric signal and sampling the converted electric signal. The interferometer calculates a displacement of the measurement surface based on the sampling value acquired by the sampling unit and a center wavelength of the laser beam. Thus, a laser source for emitting a laser beam having a highly stable center wavelength is required.
As such a laser source, an iodine-stabilized laser source has been known (for example, Document 2: JP-A-2001-274495).
The iodine-stabilized laser source disclosed in Document 2 detects a saturable absorption line of iodine by modulating the laser beam in accordance with a modulation signal and emits the laser beam while stabilizing a center wavelength of the laser beam.
The modulation signal is superimposed on the emitted laser beam, so that the wavelength of the laser beam slightly fluctuates around the center wavelength. When a sampling value is acquired by a laser interferometer having such a laser source (hereinafter referred to as a modulated laser source) while a wavelength of the laser beam is different from a center wavelength, calculation error of a displacement of the measurement surface is caused and thus measurement error of the displacement of the target object is resulted.
Thus, an iodine-stabilized laser source with external modulation in which a modulated signal is not superimposed on an emitted laser beam (hereinafter referred to as non-modulated laser source) may be as applied to a laser interferometer. However, the non-modulated laser source is more expensive than the modulated laser source and is required to be connected to a modulation element such as an AOM (acousto-optic modulator) and an EOM (electro-optic modulator) for external modulation. Accordingly, an arrangement of a laser interferometer becomes complicated.
Therefore, it has been desired to reduce a measurement error in a laser interferometer having a modulated laser source.
In this context, a sampling value may be acquired when a wavelength of the laser beam is at a center wavelength by synchronizing a modulation signal of a laser source to reduce measurement error (hereinafter referred to as frequency synchronization method).
FIGS. 11A and 11B are graphs each showing a relationship between a timing signal for acquiring a sampling value and a modulation component contained in the laser beam. In FIGS. 11A and 11B, a graph GM represents the timing signal and a graph GL represents the modulation component contained in the laser beam. In addition, a vertical axis represents a voltage of the timing signal and a deviation of a wavelength of the laser beam from a center wavelength (hereinafter referred to as wavelength deviation) and a horizontal axis represents time.
The wavelength of the laser beam is varied in a cycle TL by the modulation component contained in the laser beam.
Incidentally, a laser interferometer performs sampling at the rise of the timing signal from an L level to an H level.
The interference light is sampled in synchronization with the timing signal while the modulation cycle TL of the laser beam is equal to a cycle TT of the timing signal. When a phase difference between the timing signal and the modulation component contained in the laser beam is 0 degree, the laser interferometer can acquire a sampling value when a wavelength of the laser beam is at a center wavelength as shown in FIG. 11A.
However, when the phase difference between the timing signal and the modulation component contained in the laser beam is 90 degrees, for instance, the laser interferometer cannot acquire a sampling value when the wavelength of the laser beam is at the center wavelength.
The cause of the phase difference between the timing signal and the modulation component contained in the laser beam is attributed to, for instance, the phase difference between the modulation signal and the modulation component contained in the laser beam on account of frequency characteristics of an element modulating the laser beam in the modulated laser source or the phase difference between the timing signal and the sampling value on account of a delay time of an ADC (analog to digital converter) for sampling the interference light in the sampling unit. Incidentally, the phase difference caused by the delay time of the ADC will not be considered in the following description.
According to the frequency synchronization method, when there is a phase difference between the timing signal and the modulation component contained in the laser beam, the sampling value is acquired when the wavelength of the laser beam is not at the center wavelength, thus failing to reduce the measurement error.